mediabrat

Not saying I'm opposed to this, but the moderation logs in the Admin CP should tell you who deleted it and from where.

How active is your board? Would it be overkill to set registrations to admin validation for a while, at least until the current spam wave dies down? What if you put new members into a different group that you have set to moderate posting for the first few posts and promotes them to the regular members group afterward? Granted, it would affect all registrations, not just GMail, so it could be overkill depending on your needs, but it is one way of dealing with it.

It's possible that there's vulnerabilities in cPanel or some other server software that the hackers are exploiting. Unfortunately, if you're in a position where you don't control that level of the server (e.g. you're on shared hosting), there's not much you can do except ask your host to upgrade their servers or find a new host.